The three human colon carcinoma cell lines established at our center were studied for identification of new potential markers of differentiation. Three types of markers were investigated; these include glycogen, the Na ion- K ion- and Mg ion-stimulated adenosine triphosphatases (ATPase), and 11 purine metabolizing enzymes. The levels of these markers were determined in the clone A and clone D cells of the heterogeneous DLD-1 line, in HCT-15 cells, and in DLD-2 cells, and in their corresponding xenograft tumors in nude mice. Cultured clone A, clone D, HCT-15, and DLD-2 cells were treated with either N, N-dimethylformamide (DMF) or sodium butyrate in an attempt to modulate levels of glycogen and the purine metabolizing enzymes. Butyrate induced a significant increase in the level of glycogen in all cell types. Both DMF and butyrate modulated the activities of the various purine enzymes. The direction and extent of modulation was agent-dependent, enzyme dependent, and cell-line dependent. Of potential interest is the greatly reduced adenosine deaminase activity determined in clone A or clone D cells following exposure to DMF. Experiments testing DMF in vivo against human colon tumor xenografts in nude mice were initiated. Toxicity levels were determined, and experiments are now in progress to determine whether the colon xenografts will respond to DMF treatments.